Function duplicate_key_ids 

pub(crate) fn duplicate_key_ids<T>(
    mappings: &BTreeSet<impl MappingBackendKeyId<T>>,
    filter: &T,
    key_type: Option<String>,
) -> Option<String>
where
    T: BackendKeyIdFilter,

Collects all duplicate backend key IDs.

Accepts a set of MappingBackendKeyId implementations. Allows passing in an implementation of BackendKeyIdFilter as filter. Optionally, a key_type can be passed in which is used to complete the sentence “the duplicate{key_type} key ID”.